The initial fetal human retinal vasculature develops by vasculogenesis

There is increasing evidence that the hemangioblast, a common progenitor for hematopoietic cells and endothelial cells, participates in embryonic and extra-embryonic vasculogenesis in some organs. Whether resident angioblasts or endothelial progenitor cells (EPCs) contribute to human retinal vasculogenesis is still a matter of controversy. To address this controversy, fetal human retinas of 6-23 weeks gestation (WG) were examined using immunohistochemistry and a panel of antibodies against endothelial cell markers (CD34, CD31), a marker for retinal angioblasts and endothelium (CD39/ecto-ADPase), and a marker for precursors and hemangioblasts (CXCR4). Confocal microscopic spectral analysis and double labeling with Ki67 was used to identify the proliferating cell types. In the inner neuroblastic layer of the 6-8 WG retina and in the putative ganglion cell layer in avascular regions of older eyes (14 WG-20 WG), scattered CD39⁺ angioblasts were well in advance of forming vasculature. There was a layer of CXCR4⁺ cells in the inner retina that was reduced in size with development. As blood vessels formed, CD39 ⁺ cells were always well in advance of the vascular front and they expressed CXCR4. This demonstrates that a pool of resident angioblasts express CD39 and CXCR4 as they differentiate and participate in vasculogenesis in the fetal human. They retain expression of CD39 as endothelial cells in the newly formed retinal vasculature but they down-regulate CXCR4 expression.